1. Field of the Invention
The present invention relates generally to the fields of synthesis of polyacrylonitrile particles in an aqueous dispersion polymerization with a ionic comonomer or "surfmer" to attain essentially spherical polyacrylonitrile polymer particles with a narrow and predetermined particle size distribution. The mean particle size is directly proportional to the concentration of ionic comonomer, to the particular ionic comonomer selected, and to the counterions associated with the ionic comonomers, persulfate initiator, and bisulfite activator. This process facilitates control of polymer particle size when the polymer must contain an elevated number of dye sites. An elevated number of dye sites facilitates attaching functional molecules to the polymer, and it allows aqueous dispersion polymerization process to proceed at far lower polymer to water ratios than do normal aqueous polymerization processes. If too great a concentration of particular ionic comonomers are incorporated, the polymer particles may be too small to be efficiently filtered and must be separated by more costly centrifugal methods. At the same time, there are applications for polyacrylonitrile particles of a small uniform diameter. Polyacrylonitrile particles below 5 microns have utility in battery applications. The polymer particle is useful after conversion to a carbon powder for lithium-ion-carbon battery formulations. The important features of the polymer particles for use in advanced battery applications are a small mean particle size of less than 5 microns, a narrow particle size distribution, an essentially spherical shape, and low sodium content.
2. Description of Related Art
Acrylonitrile and its comonomers can be polymerized by a number of well-known free-radical methods. All commercial processes are based on free radical polymerization because it gives the combination of polymerization rate, ease of control, and properties including whiteness, molecular weight, linearity, and the ability to incorporate desired comonomers and, in most cases, dye sites. By far the most widely used method of polymerization in the acrylic fibers industry is aqueous dispersion polymerization or suspension polymerization. Aqueous dispersion polymerization, a variant of suspension polymerization, is the most common commercial method. Water is the continuous phase. Water acts as a convenient heat transfer and cooling medium and the polymer is very easily recovered by filtration or centrifugation. The initiators and dispersants used in dispersion polymerization are soluble in water, while those in suspension polymerization are water insoluble. Polyacrylonitrile particles made by these processes grow primarily by agglomeration of smaller particles.
Emulsion polymerization, on the other hand, is used primarily where a high level of a water-insoluble monomer is used and therefore the propagating macroradicals are isolated from each other. Encounters between macroradicals are hindered as a consequence and termination reactions are less frequent. In this process agglomeration of smaller particles is less a factor in polymer particle growth. Therefore the particle size distribution is more a function of the initial micelle size. However, it is difficult to incorporate water soluble comonomers into the polymer in an emulsion polymerization. In addition, the composition of the polymer will often change as monomers are selectively incorporated into the polymer and subsequently become deficient in the adhering monomer layer.
Acrylonitrile polymers are made from polymers of acrylonitrile that usually contain other comonomers. Nearly all acrylic fibers are made from acrylonitrile copolymers containing one or more additional monomers that modify the properties of the fiber. Neutral comonomers including methyl acrylate, methyl methacrylate, or vinyl acetate are used to modify the solubility of the acrylic copolymers in spinning solvents such as dimethyl acetamide, to modify the acrylic fiber morphology, and to improve the rate of diffusion of dyes into the acrylic fiber. Despite its disadvantages of low reactivity and difficulty in polymer control and chain transfer in polymerization, vinyl acetate is increasingly the comonomer of choice for acrylic fibers, primarily because of its low cost.
Dyes can attach to the polymer at end groups and where ionic functional groups are available. Fiber dyeability is critically dependent on the molecular weight distribution of the polymer because most acrylic fibers derive their dyeability from sulfonate and sulfate initiator fragments at the polymer chain ends. Thus, the dye site content of the fiber is inversely related to the number average molecular weight of the polymer and very sensitive to the fraction of low molecular weight polymer. With the trend to finer denier fibers where more dye is required to achieve a given color, the need for dye sites is increased. Over the years, many producers have gradually lowered the molecular weight of their polymer increase dyeability. The total number of dye sites required to be able to dye a full range of shades with cationic dyes is 30 to 50 milliequivalents per kilogram (meq/kg) depending on the fiber denier and structure. Dry-spun fibers and microdenier fibers require a minimum of 40 meq/kg of dye sites.
Where the number provided by the end groups is inadequate, a sulfonate-containing monomer may be used to provide additional dye sites within the polymer structure. Carboxylic monomers have also been employed as dye receptors. Ionic comonomers such as sodium p-styrenesulfonate, sodium methallyl sulfonate, sodium p-sulfophenyl methallyl ether, sodium 2-methyl-2-acrylamidopropane sulfonate, or itaconic acid may be added to provide dye sites apart from end groups and to increase the hydrophilic character. These dye site comonomers contain sulfonate or carboxylate functional groups and a hydrocarbon functional group. They therefore have some capacity to act as surfactants. The name surfmer, which is another term for surfactant monomer or ionic monomer, has been coined for surfactant molecules that also act as a comonomer that is able to participate in a propagation reaction.
These materials react to produce polymeric chains that fold upon themselves to form spheres. The spheres are generally several tens of microns in diameter, primarily due to agglomeration of smaller particles. Agglomeration can be partially controlled by the use of surfactants or ionic monomers. Surfactants and ionic monomers generally include counterions, often a cation such as sodium.
Polyacrylonitrile particles of various sizes have utility in a variety of applications. It is known in the art to produce carbon materials, typically fibers, by carbonization of acrylic or acrylonitrile polymers in fiber form. It has recently been discovered that acrylic polymers in small particulate form (1.5 to 4.5 microns) may be carbonized to form a material useful in battery applications. In the carbonization process, uniformity of the particle shape is essential for good result; however, prior art microparticle acrylics are made simply by crushing bigger particles and therefore have no uniformity of shape. In addition, sodium ions are detrimental to this carbonization process. Surfactants absorbed onto the polymer particles cause sodium ions to also be attached to the polymer particles.